Lubricating oils perform the following tasks under a variety of different operational conditions: (1) preventing wear and fatigue; (2) controlling friction; (3) maintaining clean systems; and (4) providing the proper viscosity to form oil films to protect surfaces.
To receive a particular SAE oil grade specification (e.g., SAE 5W-30), a lubricating oil is required to meet a set of benchmarks for a battery of tests related to its viscosity at different temperatures. Tests include the KV100, Cold Crank Simulator (CCS), High Temperature High Shear (HTHS). An additional test that was added more recently is the Mini Rotary Viscometer (MRV) test, defined by ASTM D4684-14.
The MRV test was developed in response to a particular set of field issues seen in Sioux Falls, N. Dak. in the mid-1990s, where a large number of engine failures occurred after a particular weather cooling cycle was experienced. Under these conditions, engines turned over as expected when users started their cars, but soon afterwards catastrophic damage occurred. It was determined that the root cause of the engine failure was from wax/gel structure forming in the sump and inability of the oil pump to overcome the high viscosity on the suction side of the pump where low shear rates are present. As a result, no lubricant was being fed to engine components which eventually led to seizure of the engines. The MRV test was thereafter developed to address the potential for this type of failure in lubricants, which were otherwise meeting industry standards at the time.
The methodology for the MRV test is provided by ASTM D4684-14, which is herein incorporated by reference in its entirety and for all purposes. This test method covers the measurement of the yield stress and viscosity of engine oils after cooling at controlled rates over a period exceeding 45 hours to a final test temperature of between −10° C. and −40° C. The precision is stated for test temperatures from −40° C. to −15° C. The viscosity measurements are made at a shear stress of 525 Pa over a shear rate of 0.4 s−1 to 15 s−1. The viscosity as measured at this shear stress was found to produce the best correlation between the temperature at which the viscosity reached a critical value and borderline pumping failure temperature in engines.
When an engine oil is cooled, the rate and duration of cooling can affect its yield stress and viscosity. In this laboratory test, a fresh engine oil is slowly cooled through a temperature range where wax crystallization is known to occur, followed by relatively rapid cooling to the final test temperature. These laboratory test results have predicted as failures the known engine oils that have failed in the field because of lack of oil pumpability. These documented field failing oils all consisted of oils normally tested at −25° C. These field failures are believed to be the result of the oil forming a gel structure that results in either excessive yield stress or viscosity of the engine oil, or both.
To perform the test, an engine oil sample is held at 80° C. and then cooled at a programmed cooling rate to a final test temperature and held for a specified time period. At end of this period, a series of increasing low torques are applied to the rotor shaft until rotation occurs to determine the yield stress, if any is exhibited. A higher torque is then applied to determine the apparent viscosity of the sample.
The viscosity grades of lubricating oils are determined by the Society of Automotive Engineers (SAE). Multigrade lubricating oils must fulfill two viscosity specifications, their viscosity grade consists of two numbers, e.g. 10W-40: 10W (the first number) refers to the low-temperature viscosity (“Winter Grade”), 40 (the second number) refers to the high-temperature viscosity (“Summer Grade”). Table 1 shows the specified SAE Winter Grades and the corresponding temperatures at which lubricating oils' viscosity are tested. To receive a particular grade, the lubricating oil's viscosity must be less than 60,000 cP at the listed temperature.
TABLE 1MRV Test Viscosity <60,000 cP at test temperature forSAE Winter Grade SpecifiedSAE Winter Grade0W5W10W15W20WTest Temperature (° C.)−40−35−30−25−20